Transmission lines are important elements in microwave circuit applications. These devices provide the interconnection between active and passive devices of microwave circuits, and are utilized as impedance matching elements as well. A microstrip line is a type of transmission line widely utilized in monolithic microwave integrated circuit (MMIC) applications.
Microstrip lines have a number of advantages when utilized in MMIC applications. First of all, since microstrip lines are formed of conductive planes disposed on substrates, these devices are readily adaptable to the manufacturing process of the integrated circuits. Accordingly, microstrip lines may be integrated on a same substrate with commonly used integrated circuits such as CMOS circuits.
FIG. 1 illustrates a conventional microstrip line 2 disposed over substrate 4. Microstrip line 2 includes signal line 6, ground plane 8, which is a solid metal plane, and dielectric layer(s) 10 separating signal line 6 from ground plane 8. Ground plane 8 has the advantageous feature of signally isolating signal line 6 from substrate 4, and hence the substrate-induced losses are reduced. However, the formation of ground plane 8 also incurs drawbacks. As the backend processes are scaled down, the vertical distance H between signal line 6 and ground plane 8 becomes significantly smaller, hence requiring signal line 6 to be increasingly narrower in order to achieve the desirable characteristic impedance. Consequently, Ohmic losses in microstrip lines become increasingly more significant, thus demanding better impedance matching between microstrip line 2 and other network devices. Further, limited by the vertical distance H between signal line 6 and ground plane 8, which distance has little room for tuning, ground plane 8 itself becomes a barrier for tuning the characteristic impedance of microstrip line 2.
In addition, microstrip lines typically occupy great chip area. For example, the electro-magnetic wavelength in SiO2 dielectric material is about 3000 μm at 50 GHz. Accordingly, microstrip line 2, with the requirement that its length L1 needs to be at least a quarter of the wavelength, which is about 750 μm, in order to match network impedance, is area-consuming. With the increasing down-scaling of integrated circuits, the chip-area requirement of the microstrip lines becomes a bottleneck preventing the integration of microwave devices and the integrated circuits adopting complementary metal-oxide-semiconductor (CMOS) devices.
Accordingly, what is needed in the art are transmission lines that may take advantage of the benefits associated with the reduced energy losses while at the same time overcoming the deficiencies of the prior art.